1. Field of the Invention
The present invention relates to implantable stimulators, which deliver electrical stimulation pulses to tissue of an animal for therapeutic purposes, and more particularly to the waveforms of such electrical stimulation pulses.
2. Description of the Related Art
A remedy for people with slowed or disrupted natural heart activity is to implant a cardiac pacing device which is a small electronic apparatus that electrically stimulates the heart to beat at regular rates.
Typically a battery powered pacing device is implanted in the patient's chest and has sensor electrodes that detect natural electrical impulses associated with in the heart contractions. These sensed impulses are analyzed to determine when abnormal cardiac activity occurs, in which event a pulse generator is triggered to produce artificial electrical pulses. Wires carry these pulses to stimulation electrodes placed adjacent specific cardiac muscles, which when electrically stimulated contract the heart chambers. It is important that the electrodes be properly located to produce contraction of the heart chambers.
Modern cardiac pacing devices vary the stimulation to adapt the heart rate to the patient's level of activity, thereby mimicking the heart's natural action. The pulse generator modifies that rate by tracking electrical signals at the sinus node of the heart or by responding to other sensor signals that indicate body motion or respiration rate.
The waveforms of the stimulation pulses are integral to the pacing process and are a function of the characteristics of a pacing signal generator; the electrical leads connecting that generator to the pacing site; the contact interface between the lead and the pacing site; and physiological and electrical characteristics of the tissue to be stimulated. FIG. 1 illustrates a traditional rectangular conventional pacing pulse CP that is characterized by a nominal amplitude VS0 that is “on” for a duration TP0 of about 0.4 ms to 2.0 ms. The integral of the waveform pulse is denoted by area “A0” under the pulse.
In this context, the overall system impedance, including that of the tissues, is complex with both reactive and resistive components. Since the generator load impedance is reactive, a square waveform in the time-domain at the signal generator degenerates to a composite of exponential rise and decay curves at the pacing site. These waveforms are filtered by the tissue impedance wherein higher frequency components get attenuated at the pacing site. Therefore, for short timed waveforms, the effective pacing amplitude at the pacing site becomes reduced.
In order to stimulate tissues, the initial rate of change of voltage (dV/dt) (voltage slope) has an impact on pacing effectiveness. A faster rising waveform will stimulate sooner than a slowly rising waveform, even when the final pacing waveform amplitudes are the same at the signal generator. In the present context, due to the time constants involved, the waveform measured at the pacing site lags the waveform at the generator. As a consequence, fast rise and fall times at the signal generator appear significantly attenuated with slower slopes at the stimulation site. When the waveforms are very short in duration, the effect of the lagging results in the amplitude at the pacing site never reaching a final steady state amplitude, as the waveform returns to zero before the maximum amplitude is reached.
Designers of prior art systems, kept the overall stimulation current reduced by increasing the resistance of the electrical leads, which limited the peak current from the pacing generator. However, this approach also reduced the efficiency of the pacing system.
Prior tissue stimulation devices occasionally had a side effect of stimulating nerves in the vicinity of the primary site which resulted in muscle twitching that was very uncomfortable to the patient.
From the prior examples, there is a need for a stimulation method that has improved pacing efficiency in a manner that does not causes collateral nerve stimulation.